I just started playing Space Marines but I'm having a bit of trouble with figuring out what I want to do with my Strike Cruisers. Generally speaking it seems like they're glorified light cruisers and my experience has been that they don't tend to last long in a straight fight, but on the other hand their guns are relatively short-ranged. Of course, boarding torpedoes and Thunderhawks are a thing. How are you guys building your Strike Cruisers?

Anything that makes them more survivable is always good, so shield upgrades and the like. Obviously either boarding torps or squadron refuelling depending on if you went for the torpedo launchers or hangars (personal preference for me is the Mk1 cruiser with the launch bays).

Further mods depends on how you want to play. If you're getting in their faces with as many boarding actions as possible, Chapter Banner is useful. Focus on weaponry to try and reduce the return fire, and focus on the ships with the most AP potential. Against Chaos you're going to struggle because of how many lances and hangar bays they have, especially if they're fielding far more ships than you, and ESPECIALLY if they have Nurgle or Slaanesh favours. In those cases, you either die gloriously for the Emperor or warp out.

Your other option is to kite with boarding torp spam and recon beacons. Upgrades to augur range could be useful, but you really want to be further away than 7.5k. Its not really a playstyle I enjoy or have much experience with I'm afraid.

I actually stay away from boarding torpedo except on my battlebarges and even then they are an auxiliary thing. I equip all of my ships that I use with Advance Cogitator Links and Belt Armor; the first as special orders are amazing as space marines, the second because strike cruisers are basically worthless if the wrong system gets blown (namely prow or engines). Auxiliary Shield Capacitors are better than Additional Void Shield Generator in my opinion, though if you do a lot of hit and runs giving your self time out of combat to recharge your shields then Additional Void Shield Generator is the way to go. Efficient Plasma Thrusters are a common upgrade for me as well. One of the greatest strengths that space marines have is that they can dance around enemies with strike cruisers and vanguards while letting the battlebarge brawl it out. Maneuvering Thrusters(you want to dodge around ships more or quickly angle yourself for a torpedo volley), Voss Pattern Void Shields(only if you have another shield upgrade), Turbo Weaponry(if you want a support cruiser, usually a Mk.I), and Chapter Banner(you want to board more often) are all situational. I see a lot of SM players in ranked using boarding torpedoes nearly exclusively, however I've found that Bombardment Cannons are the greater strength of the fleet. With 3 points in Master Gunner, and Lock On you can get 78.75% crit chance (98.4375% with maxed out Chapter Master in the fleet).